The invention relates to an optical coupling device for injecting light between two optical waveguide end faces, the geometric position of one optical waveguide end face, for example of an optical fibre, being capable of being varied with respect to the other optical waveguide end face, for example of a strip conductor of an optical component, with the aid of a variable-length element, which carries one of the two optical waveguides via a holding element and is connected to the structure containing the other optical waveguide, or is fixed to said structure, by means of at least one holding block.
An optical coupling device is disclosed, for example, by WO 98/13718. Coupling devices of this type are used in optical filters based on the phased-array principle having an input coupling face which light enters at a specific geometric position, the geometric position influencing the output wavelength of the optical filter. Optical filters based on the phased-array principle are used in particular as multiplexers or demultiplexers in optical wavelengths multiplex operation (WDM), since they exhibit low insertion attenuation and high crosstalk suppression.
German patent application DE 44 22 651.9 describes how the centre wavelength of a phased-array filter can be defined by the position of an input coupling optical waveguide which leads the light into the optical waveguide. In this way, the centre wavelength of the optical filter can be adjusted exactly by means of the geometric positioning of the input coupling optical waveguide or the input coupling fibre.
Optical coupling devices are also used in narrow-band wavelength multiplexers (DWDM) for optical waveguide transmission technology. These components make it possible, on the transmitter side, to combine the signals from lasers of various wavelengths with low losses onto a single glass fibre or, respectively, on the receiver side, to divide said signals to a corresponding number of receivers in a wavelength-selective manner.
The particular advantage of narrow-band wavelength multiplexers as compared with conventional wavelength multiplexers lies in their narrow-band nature. As a result, such a small channel spacing is possible that, at the minimum attenuation of the glass fibre, that is to say in the wavelength range around 1550 nm, a large number of transmission channels, for example 32 transmission channels, can be accommodated. A DWDM comprises a chip, to which waveguide structures with the necessary geometry are applied. On the receiver side, the input of the chip is the fibre with the multiplexer signal, which is also referred to as the input coupling fibre. On the output coupling side, a corresponding number of fibres are fitted, which lead the individual signals on to the receiver.
In optical waveguide transmission technology with DWDM, the problem is that the characteristics of the chip change sharply with the operating temperature. A temperature change leads to a change in the refractive index relationships and also the geometric relationships of the chips. As a result, wavelength shifts occur, that is to say the branch of the channel between DWDM and the lasers and, respectively, between the transmitter side and the receiver side is shifted. For this reason, the shift in the centre wavelengths must be avoided.
In order to avoid the temperature effects described, passive temperature compensation has already been proposed. The temperature dependence of the centre wavelength can be compensated for by the fact that the input coupling fibre is shifted vertically with respect to the DWDM chip as a function of the temperature. This shift is carried out by means of a variable-length component which, as compared with the carrier material of the chip, has a higher thermal coefficient of expansion, for example by means of a variable-length element made of aluminium. Then, as was described at the beginning, the optical fibre is fixed to the variable-length element, so that the end faces of the optical fibre and of the optical conductor chip (also called a planar waveguide or a planar waveguide chip) are shifted parallel to one another, which compensates for the influence of the temperature on the centre wavelength.
In the practical implementation of this coupling device, the connecting points between the holding block and the chip, on the one hand, and the holding block and the variable-length element, on the other hand, are designed using adhesive bonding technology. In this case, the bonding point between the holding block and the chip is cured after the input coupling fibre has been positioned optically relative to the chip.
In the case of this technology, the problem arises that the bonded connections are subject to temperature-dependent changes. As a result of different bond gap widths, inhomogeneities and gassing-out of the adhesive, mechanical stresses arise in the gap. This is particularly critical in the case of bonded connections between materials with different coefficients of thermal expansion, such as aluminium and glass or glass ceramic. The consequence of the thermally inducted stresses is that a temperature change not only affects the desired movement of the end faces of the optical conductor elements in relation to each other, but also movements perpendicular thereto, that is to say perpendicular to the plane of the chip or away from the chip. These movements are undesired, since they lead to an increase in the attenuation at the input coupling point. The undesired movements may be prevented, at least partially, by means of fixing the free end of the variable-length element, but the fixing has to be configured in such a way that the desired temperature-dependent movement is permitted.
It has already been proposed to provide a displaceable guide on the other holding block. However, this type of fixing requires very close machining tolerances of the components and a great deal of precision mechanical effort. Nevertheless, problems arise as a result of friction and play in the guide.
By contrast, the invention provides an optical coupling device in which movements of the end face of the optical fibre which movements are perpendicular to this face are suppressed and, at the same time, the desired movement of the end face of this optical fibre parallel to the end face of another optical fiber or waveguide such as a planar waveguide, is permitted. In particular, an optimal coupling device is to be provided which is compatible with the established manufacturing and adhesive bonding methods and permits adjustment of the input coupling point before adhesive bonding.
Further, the optical coupling device mentioned in the Field of the Invention contains, among other elements, a variable-length element which is connected to a variable-length compensating element, whose length changes with the temperature by the same amount but in the opposite sense as that of the variable-length element, and in that the variable-length compensating element is fixed to a second holding block.
The variable-length element, which can consist of aluminium, for example, in this embodiment of the invention is lengthened by a compensating element made of a material with a negative coefficient of expansion, so that the result overall is the same thermal expansion as in the carrier material, for example quartz glass. As a result, although the input coupling fibre is shifted in the desired manner, that is to say the end face of the input coupling fibre moves parallel to the input coupling face of the chip, no relative movement takes place between the fixing points of the two holding blocks and the carrier material, that is to say the chip, since the total length of the variable-length element and variable-length compensating element is always of the same size. Therefore, the stresses and shifts described above are minimized.
A further advantageous refinement of the device according to the invention is that the length of the variable-length compensating element is selected, taking its coefficient of expansion into account, such that the length of the variable-length compensating element changes by the same amount but in the opposite sense or amount as that of the variable-length element. In other words, only the combination of the influences of the length of the compensating element and its coefficient of expansion matters, so that accurate matching of the coefficient of expansion is not necessary.
In the optical coupling device first mentioned in the Field of the Invention, the holding block has a U-shaped part made of a material with the same coefficient of thermal expansion as the chip, in that a T-shaped part made of a material with the same coefficient of thermal expansion as the chip is provided, and in that the variable-length element with the positive coefficient of thermal expansion is connected to the T-shaped part at its foot and to the U-shaped part at its base, and in that two variable-length elements with a positive coefficient of thermal expansion are fixed to the legs of the U-shaped part, which consist of the same material as the variable-length element and have the same length as the latter, and which, on one side, are fixed to the legs of the U-shaped part and, on the other side, to the underside of the crossbar of the T-shaped part. This coupling device is fixed or adhesively bonded to the chip with the aid of the U-shaped part. By means of the identical thermal expansions of the three columns formed by the variable-length elements and the U-shaped and the T-shaped part, permanent adhesive bonding of the individual parts is made possible, without the connection points being stressed as a result of temperature expansions. Therefore, the input coupling fibre can advantageously carry out the desired temperature-dependent movements. As a result of the additional parts, the desired fixing of the upper end of the variable-length element is achieved, so that temperature-dependent and time-dependent changes in the adhesive bonding point between the U-shaped part and the variable-length elements can have only a minimal effect. Only the U-shaped part is connected or adhesively bonded to the chip, and all the other parts can move freely and, therefore, are able to shift in the event of fluctuating temperatures and corresponding expansion of the variable-length elements with a positive coefficient of thermal expansion.
A further advantageous refinement of the device according to the invention is that the variable-length elements consist of aluminium which, because of its material characteristics, is preferred for this purpose.
Finally, a further advantageous refinement of the device according to the invention is that the material of the variable-length compensating elements is a glass ceramic with a negative coefficient of thermal expansion, preferably the material of the chip. This achieves a minimum influence of temperature changes between chip and holding block.